cofdm dtv.pdf
TRANSCRIPT
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17.4.2008 Digital televsion techniques Lecture 7 1
Lecture 7
Digital television
Coded Orthogonal Frequency Division Multiplexing
Need for a good transmission techniqueExplanation of the different parts
CodedFrequency Division MultiplexingOrthogonal
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17.4.2008 Digital televsion techniques Lecture 7 2
DVB-S
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17.4.2008 Digital televsion techniques Lecture 7 3
DVB-C
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17.4.2008 Digital televsion techniques Lecture 7 4
DVB-T
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17.4.2008 Digital televsion techniques Lecture 7 5
DVB-S/C/T Comparison
XBaseband shaping / QAM modulationXDifferential encodingXByte to m-tuple conversion
XOFDMXXMapping and modulation (QAM / QPSK)XXInner interleavingXXInner coding
XXXOuter interleavingXXXOuter coding (Reed-Solomon (204,188))XXXEnergy dispersalTCS
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17.4.2008 Digital televsion techniques Lecture 7 6
Design goals for digital terrestrialvideo broadcasting
Single Frequency Network (SFN)Mobile receptionProblems
Multipath interference - ghostsNoise interference snowVariable path attentuation fadingInterference to existing servicesInterference from other services
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17.4.2008 Digital televsion techniques Lecture 77
Mobile receptionProTeleVision (Denmark) builds SFN network in Singapore
Mobile Television onboarda tram in Cologne, Germany
Mobile television in Turku bus
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17.4.2008 Digital televsion techniques Lecture 7 8
Developed by the digital video broadcasting project group - DVBUses similar technology to DRB (DAB)Uses 1705 or 6817 carriersVariable carrier modulation types are defined allowing data rates of 5-27 Mb/s in 7 MHzDeveloped for 8 MHz channels
A 7 MHz variant has been produced and testedCan use single frequency networks - SFNsNew technology with scope for continued improvement & development
Mobile reception
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17.4.2008 Digital televsion techniques Lecture 7 9
BST-OFDM is a variant of the European COFDM system which allows segmenting of the data spectrum into 100 kHz blocks.2 receiver bandwidths proposed.
500 kHz portable / mobile for sound and data5.6 MHz fixed / mobile for SDTV and LDTV5.6 MHz fixed for HDTV
Individual band segments can be allocated to separate services which can use different modulation systems
BST OFDM Japan
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17.4.2008 Digital televsion techniques Lecture 7 10
Existing analog TV channels are spaced so they do not interfere with each other.Gap between PAL TV services
VHF 1 channelUHF 2 channels
Digital TV can make use of these gapsCh 7 Ch 8 Ch 9Ch 6 Ch 9A
VHF Television Spectrum
Channel spacing
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17.4.2008 Digital televsion techniques Lecture 7 11
Digital TV must co-exist with existing PAL services
DTV operates at lower powerDTV copes higher interference levelsShare transmission infra-structureDTV needs different planning methods
Ch 7 Ch 8 Ch 9Ch 6 Ch 9A
VHF Television Spectrum
8-VSB COFDM
Digital challenges
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17.4.2008 Digital televsion techniques Lecture 7 12
The effect of a multicarrier system
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17.4.2008 Digital televsion techniques Lecture 7 13
Mathematical description of COFDM
Each carrier is modulated
Several carriers are summed
Takes fixed values
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17.4.2008 Digital televsion techniques Lecture 7 14
Mathematical description of COFDM
Zeroth frequency -> 0 gives
Compare with IFT
Equivalent if
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17.4.2008 Digital televsion techniques Lecture 7 15
Visualization of COFDM
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17.4.2008 Digital televsion techniques Lecture 7 16
Modulation of subcarriers
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17.4.2008 Digital televsion techniques Lecture 7 17
COFDM principle
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17.4.2008 Digital televsion techniques Lecture 7 18
Frequency
MinimumCarrier Spacing
Traditional SCPC Modulation
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17.4.2008 Digital televsion techniques Lecture 7 19
COFDM Orthogonal carriers
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17.4.2008 Digital televsion techniques Lecture 7 20
COFDM Orthogonal carriers
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17.4.2008 Digital televsion techniques Lecture 7 21
6.67 MHz in 7 MHz Channel
AlmostRectangular
Shape
1705 or 6817 Carriers
Carrier Spacing2k Mode 3.91 kHz8k Mode 0.98 kHz
Spectrum of COFDM
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17.4.2008 Digital televsion techniques Lecture 7 22
Subchannel response - pilots
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17.4.2008 Digital televsion techniques Lecture 7 23
Subchannel response - pilots
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17.4.2008 Digital televsion techniques Lecture 7 24
Subchannel response - pilots
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17.4.2008 Digital televsion techniques Lecture 7 25
AnalogDigital
Good
RottenClose FarDistance
Quality
Edgeof
ServiceArea
System failure characteristics
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17.4.2008 Digital televsion techniques Lecture 7 26
COFDM parameters
Carrier modulation: 2k, 8kType of modulation QPSK, 16QAM, 64QAMGuard interval , 1/8, 1/16, 1/64Inner coder puncture rates: , 2/3, , 5/6, 7/8Hierarchical modesSelection of transmission bandwidth (6/7/8 MHz)
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17.4.2008 Digital televsion techniques Lecture 7 27
DVB in Finland / OthersParameters: FINLAND :COFDM, 8k, 64QAM, 2/3 Code, Guard interval 1/8
SWEDEN: COFDM, 8k, 64QAM, 2/3 Code, Guard interval 1/8
UK: COFDM, 2k, 64QAM, 2/3 Code, Guard 1/32
ITALY: COFDM, 8k, 64QAM, 2/3 Code, Guard 1/4COFDM, 8k, 64QAM, 3/4 Code, Guard 1/32
A+B
C
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17.4.2008 Digital televsion techniques Lecture 7 28
DVB-T Parameter selection
Number of carriers (2k / 8k)Intercarrier spacing is a function of number of carriers
More carriers: More sensitive to frequenqy offsets, less sensitive to maximum delay spread
Type of modulationHigher order: More bits on air, more sensitive to noise
Code rateCapability of correcting errors (decrease with increaseing code rate
Gard intervalLonger gard interval: Increased maximum delay spread, less data
Senstivity to freqency offset
Maximum delay spread
2k
8k
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17.4.2008 Digital televsion techniques Lecture 7 29
OFDM Planning example
8 MHz bandwidth8k system, 64 QAM, 2/3 code, GI 1/8Bandwidth: 20MHzSubcarrier spacing : f = 8MHz / 8192 = 976 HzOFDM symbol duration: TFFT = 1/ f = 1024usCyclic prefix duration: TGI = 128us (1/8)Symbol duration: Tsymbol = TFFT + TGI = 1152usSymbol frequency fsymbol = 1/Tsymbol = 868 s-1Bits per carrier (64QAM) 6Active carriers per symbol
22,7 Mbits/s
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17.4.2008 Digital televsion techniques Lecture 7 30
The DVB-T standard gives specific values used in implementations
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17.4.2008 Digital televsion techniques Lecture 7 31
DVB-S in the World
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17.4.2008 Digital televsion techniques Lecture 7 32
DVB-C in the World
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17.4.2008 Digital televsion techniques Lecture 7 33
DVB-T in the World
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17.4.2008 Digital televsion techniques Lecture 7 34
System parameters
FFT size: 64
Number of tones used 52 (12 zero tones)Number of pilots 4 (data tones = 52-4 = 48 tones)Bandwidth: 20MHz
Subcarrier spacing : f = 20MHz / 64 = 312.5 kHz
OFDM symbol duration: TFFT = 1/ f = 3.2us
Cyclic prefix duration: TGI = 0.8us (1/4)Signal duration: Tsignal = TFFT + TGI
Case study IEEE 802.11a WLAN
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17.4.2008 Digital televsion techniques Lecture 7 35
Modulation: BPSK, QPSK, 16-QAM, 64-QAMCoding rate: 1/2, 2/3, 3/4
FEC: K=7 (64 states) convolutional code
Case study IEEE 802.11a WLAN
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